In general, a compressor of a refrigerant cycle is lubricated by a lubricating oil mixed with gas refrigerant. If the oil is discharged from the compressor into the refrigerant cycle with the gas refrigerant, a coefficient of performance (hereinafter, system COP) of the refrigerant cycle is reduced. The system COP is reduced as the amount of the oil discharged from the compressor is increased. It is known to arrange a centrifugal oil separator at an outlet of the compressor in order to reduce the amount of oil discharged in the refrigerant cycle from the compressor.
For example, Japanese Unexamined Patent Application Publication No. 3-129273 (U.S. Pat. No. 5,159,820) describes an oil separator having a separation chamber and an oil storage chamber. The separation chamber is disposed at a rear part of a rear housing of a compressor. The separation chamber has an inlet opening on its side wall, a refrigerant discharge pipe at its upper portion, and an oil outlet opening. Refrigerant compressed in the compressor is introduced in the separation chamber through the inlet opening. In the separation chamber, oil is separated from the refrigerant. After the oil separation, the refrigerant is discharged from the separation chamber through the refrigerant discharge pipe, and the oil is introduced in the oil storage chamber through the oil outlet opening. The oil storage chamber is disposed under the oil separation chamber, and stores the oil therein.
Recently, CO2 refrigerant is used in consideration of environmental issues. A difference in density between the CO2 refrigerant and the oil is smaller than a difference in density between chlorofluorocarbon refrigerant and the oil. Therefore, in a conventional oil separator, it is difficult to sufficiently separate the oil from the CO2 refrigerant by means of centrifugal force. As a result, oil separation efficiency reduces. In such a case, an oil rate in a refrigerant cycle will increase, and hence efficiency of heat exchange in a gas cooler and an evaporation will reduce. Further, the system COP reduces.
To address the foregoing issue, it is considered to reduce an inner diameter of a separation cylinder defining a separation chamber in order to increase the centrifugal force. In such a case, however, a clearance between an inner surface of the separation cylinder and an outer surface of a refrigerant discharge pipe reduces. With this, if the gas refrigerant introduced in the separation cylinder collides with the discharge cylinder, the velocity of turning flow of the refrigerant reduces. As a result, the oil separation efficiency reduces.
If the oil separation efficiency reduces, the amount of oil stored in the oil storage chamber reduces. In such a case, it is difficult to supply the compressor with a sufficient amount of oil. As a result, reliability of working the compressor will reduce.